Before the advent of digital signal processing (DSP), all selectivity in a GS a radio receiver was provided by analog filters. The signal, or pass)(band, of interest was selected by analog filter devices that attenuated all other signals. These analog filter devices typically take the form of crystal networks, ceramic resonators, and more recently, Surface Acoustic Wave (SAW) devices.
Receivers using only analog signal processing are still common because of cost considerations. Digital channel filtering can be computationally expensive if it seeks to duplicate the high level of undesired signal rejection available in analog filters. A multi-bandwidth receiver using only analog filtering is described in U.S. Pat. No. 5,020,092. This receiver has two bandwidth settings which are selectable by applying the signal of interest to one of two analog filters.
As DSP costs decline, more of the channel filtering is being done with digital processing. Some solutions use almost exclusively digital processing. For example, a "Digital Downconverter" IC (HSP50016) manufactured by Harris Corp. provides all of the stopband rejection within many megahertz of the signal of interest. This is a computationally expensive solution because it requires the digital filtering to completely define the stopband at a high sample rate. It requires on-chip DSP hardware dedicated to performing the required high-rate, high-order filtering.
The combination of older analog filtering with newer digital filtering is known. U.S. Pat. No. 4,803,700, for example, teaches this combination for use in a single-bandwidth receiver, where the ultimate rejection of the stopband is defined, at various frequencies, by both analog and digital filtering for this single bandwidth. The stopband of the digital filtering still needs to be very well-defined This patent does not teach multi-bandwidth operation and is not well suited for that mode of use.
The replacement of the second intermediate frequency (IF) filter (usually 450 or 455 kHz) with DSP filtering to make multi-bandwidth operation easier is also known. Such a configuration is described in Application Note AN-301, published by Analog Devices, Inc. This configuration does not use an analog filter to define the stopband of the passband of interest. Thus, no consequent reduction in DSP processing is realized.
There therefore exists a need for a low cost, high quality selectivity radio receiver wherein multiple passbands of interest are selectively defined. It would be desirable if such a receiver could utilize the low cost selectivity associated with an analog filter with the improved frequency response associated with a digital filter.